Propylthiouracil (PTU) and Methimazole (MMI) are thioureylene derivatives with potent antithyroid effects. Their major intrathyroidal action is to block the biosynthesis of thyroid hormone by interfering with the thyroid peroxidase enzyme. In addition, PTU has major effects on thyroid hormone metabolism. Both drugs have been used therapeutically for the treatment of hyperthyroidism for three decades, but little is known about their pharmaco-dynamics or mechanism of action. We have recently developed a highly sensitive and specific radioimmunoassay for propylthiouracil. Using this assay, we will study PTU pharmacodynamics in normal human subjects as well as in those with hyperthyroidism and nonthyroidal illnesses. The sensitivity and specificity of this assay will permit an analysis of the tissue distribution and subcellular localization of PTU in the rat. In vitro studies will explore the mechanism of action of PTU, using purified lactoperoxidase (which is also inhibited by PTU), as well as partially purified thyroid peroxidase. These investigations will include the isolation of a putative drug-thyroid peroxidase complex and the elucidation of its molecular nature. The information obtained from these studies will provide preliminary background data for the purification of thyroid peroxidase by affinity chromatography. We have synthesized derivatives of PTU which can be conjugated to agarose columns and used to specifically bind thyroid peroxidase or lactoperoxidase. This technique may provide a new tool for the isolation of a thyroid peroxidase of high specific activity. Recently, PTU has been observed to have extrathyroidal actions. One of the more important effects is the ability to protect against toxic liver injury. We plan to examine this phenomenon with dose-response and structure-activity studies using PTU analogues. We will also see if PTU's protective effects extend to a murine viral hepatitis model, which would have important implications for human viral hepatitis. Finally, a radioimmunoassay for MMI will be developed using the techniques devised in our laboratory for the PTU radioimmunoassay. Pharmacokinetics, tissue and subcellular distribution, and mechanism of action of MMI will be studied.